This invention relates to channel search and selection systems for television receivers, and more particularly, to a channel search and selection system, in which channels are searched and selected according to a variable d.c. voltage supplied to an electronic tuner.
As the channel selector of a television receiver is frequenctly operated, it should be readily operable and have high reliability and durability. Electronic tuners of the television receiver do not have mechanical contacts and permit the reception of television signals of desired channels with a variable turning voltage. Thus, the channel selector using an electronic tuner permits the reception of a television signal of a desired channel through a variation of a tuning voltage caused by operating a simple pushbutton switch or touch switch. In addition to the ready operation, it is superior in reliability since it operates electronically. As a channel search and selection system using an electronic tuner, there is a voltage synthesizer system disclosed in U.S. Pat. No. 4,240,115 which is assigned to the assignee of the present invention. In this system, a tuning voltage supplied to an electronic tuner is continuously varied and, when a television signal of a channel is received, the tuning voltage is held at the value of this instant, stopping the tuning voltage variation, whereby this channel is steadily received. This voltage synthesizer system is usually provided with an automatic frequency control function. The automatic frequency control is effected as the output signal of a frequency discriminator, which detects the frequency of an i-f signal output of the electronic tuner converting a television signal thereto, is fed back to the electronic tuner. In other words, when the i-f signal from the electronic tuner does not have a frequency coincident with a predetermined frequency, the frequency discriminator supplies such an output signal to the electronic tuner as to control the tuning voltage so that the i-f signal has a frequency coincident with the predetermined frequency. So long as the coincidence of the i-f signal frequency with the predetermined frequency is obtained, the television signal is correctly and steadily received.
FIG. 1 is a block diagram of a television receiver provided with a voltage synthesizer channel search and selection system. The receiver comprises an electronic tuner 2 connected to an antenna, an i-f amplifier 3 connected to the tuner 2, a video detector 4 connected to the i-f amplifier 3, a video amplifier 5 connected to the video detector 4, a cathode-ray tube (hereinafter referred to as a CRT) 6 connected to the video amplifier 5, a sync separator 7 connected to the video detector 4, a deflection circuit 8 connected to the sync separator 7, a frequency discriminator 9 connected to the i-f amplifier 3, and a channel selection voltage generator circuit 10 connected to the sync separator 7, the deflection circuit 8 and the frequency discriminator 9. The electronic tuner 2 converts a television signal intercepted by the antenna 2 to an i-f signal which is fed to the i-f amplifier 3. The i-f amplifier 3 amplifies the i-f signal, and its output is fed to the video detector 4 and also to the frequency discriminator 9. The video detector 4 detects the i-f signal input to reproduce a composite television signal containing video signal and sync signals. The composite signal thus obtained is fed to the video amplifier 5 and also to the sync separator 7. The video amplifier 5 amplifies the video signal, and its output is fed to the CRT 6 for display as image reproduction thereon. The sync separator 7 separates the sync signal from the composite signal, and the separated sync signal is fed to the channel selection voltage generator circuit 10 and also to the deflection circuit 8. The deflection circuit 8 deflects the electron beam produced in the CRT 6, and at the same time, produces flyback pulses (i.e., retrace pulses) which are fed to the channel selection voltage generator circuit 10. The frequency discriminator 9 detects the frequency of the i-f signal to produce an output signal varying depending on whether the i-f signal frequency is higher or lower than a predetermined frequency. The detection of the discriminator 9 is fed to the channel selection voltage generator circuit 10. The function of the channel selection voltage generator circuit 10 will be described hereinafter in detail.
FIG. 2 shows the detailed circuit construction of the channel selection voltage generator circuit shown in FIG. 1. It includes a buffer circuit 11, a Schmitt circuit 12, a sync discriminator circuit 13, AND gates 14 and 15, NOR gates 16 and 17, NAND gates 18 and 19, an inverter 20, set-reset type flip-flops (hereinafter referred to as SRFF) 21 and 22, a tuning voltage generator 22, which has a capacitor 24 for delivering a tuning voltage, a first constant current circuit 25 for supplying a charging current to the capacitor 24 and a second constant current circuit 26 for causing a discharge current from the capacitor 24, pushbutton switches 27 and 28 for causing tuning operations, and resistors 29 and 30.
The sync discriminator circuit 13 receives the sync signal output b from the sync separator 7 (FIG. 1) and the flyback pulse output c from the deflection circuit 8 (FIG. 1). When these signals b and c are synchronized with each other, the sync discriminator circuit 13 produces at an output terminal X a logical level "1" signal and at an output terminal Y a logical level "0" signal, while when the signals b and c are not synchronized the circuit 13 produces a logical level "0" signal at the terminal X and a logical level "1" signal at the terminal Y. When receiving a level "1" signal from the NOR gate 17, the first constant current circuit 25 supplies a current I to the capacitor 24 for charging the capacitor 24, while when receiving a level "0" signal from the gate 17 the circuit 25 supplies no current. The second constant current circuit 26 causes a current of a value I/2 unless the terminal voltage across the capacitor 24 is zero. The voltage across the capacitor 24 is delivered from a terminal D to the electronic tuner 2. To a terminal A, the output voltage a of the frequency discriminator 9 is applied. The buffer circuit 11 produces an output signal at level "1" when the voltage a is higher than a first predetermined voltage E.sub.1, and produces an output signal at level "0" when the voltage a is lower than E.sub.1. The Schmitt circuit 12 has a hysteresis characteristic, and it produces an output signal at level "1" when the voltage a increases higher than a second predetermined voltage E.sub.2 and subsequently produces an output signal at a level "0" when the voltage a decreases lower than a third predetermined voltage E.sub.3 which is lower than the voltage E.sub.2. Usually, the first voltage E.sub.1 mentioned above is lower than the second voltage E.sub.2 and higher than the third voltage E.sub.3.
The pushbutton switches 27 and 28 are provided for selecting a channel other than that under receiption. For example, when the pushbutton 27 is turned on, a switching is effected from a first channel to a second channel, while when the pushbutton switch 28 is turned on a switching is effected from the second channel to the first channel. More particularly, when the switch 27 is turned on, the tuning voltage being delivered from the termfinal D to the electronic tuner 2 is increased, while when the switch 28 is turned on the tuning voltage being delivered to the tuner 2 is decreased.
The function of the channel selection voltage generator circuit 10 will now be described. It is assumed that the electronic tuner 2 is receiving the television signal of a certain channel with a tuning voltage delivered thereto from the tuning voltage generator circuit 23. In this state, a voltage substantially equal to the first voltage E.sub.1, i.e., the threshold voltage of the buffer circuit 11, appears at the terminal A. Also, the sync signal output b appears at terminal B, and the flyback pulse output c at terminal C, these signals being synchronized with each other. Further, the SRFFs 21 and 22 are reset so that their output signals are at level "0".
In this state, if the voltage a applied to the terminal A becomes lower than the first voltage E.sub.1 or the threshold voltage of the buffer circuit 11 at a time t, the output signal of the buffer circuit 11 is inverted to a "0" level signal, the output signal of the NOR gate 16 is inverted to a "1" level signal, the output signal of the NOR circuit 17 is inverted to a "0" level signal, and the current from the constant current circuit 25 is cut off. As a result, the capacitor 24 is discharged through the constant current circuit 26 to reduce the voltage at the terminal D. With the reduction of the voltage at the terminal D, i.e., the tuning voltage, the frequency of the i-f signal from the electronic tuner 2 is reduced to increase the output voltage a of the frequency discriminator 9. In other words, when the output voltage a becomes lower than the threshold voltage of the buffer circuit 11, the feedback loop functions to increase the voltage a, so that until the voltage a eventually reaches the threshold voltage of the buffer circuit 11, the logical level of the output signal of the buffer circuit 11 is not inverted to the "1" level to invert the output signal of the NOR gate 16 to the "0" level signal and invert the output signal of the NOR gate 17 to the "1" level signal, thus turning on the constant current circuit 25 again for charging the capacitor 24. As a result, the voltage at the terminal D, i.e., the tuning voltage, is no longer reduced and turns to increase, thus generally increasing the frequency of the i-f signal from the electronic tuner. With the gradual increase of the i-f signal frequency the output voltage a is reduced again and eventually becomes lower than the threshold voltage of the buffer circuit 11 to cut off the constant current circuit 25 again. In the above way, the voltage at the terminal D is held substantially at the constant voltage E.sub.1 with slight periodic changes, so that the electronic tuner 2 can correctly receive the electronic tuner 2. The above function is the automatic frequency control function. Of the current I from the constant current circuit 25, one half flows through the constant current circuit 26, and the other half is supplied to the capacitor 24.
The selection of a channel is effected by operating the pushbutton switch 27 or 28. When the pushbutton switch 27 is turned on, the SRFF 21 is set, and its output signal level is now at "1", thus rendering the output signal of the NOR gate 16 to be at "0" level and the output signal of the NOR gate 17 to be at "1" level. As a result, current is caused through the constant voltage circuit 25. At this time, the output of the NOR gate 17 is at "1" level regardless of the amplitude of the output signal a supplied from the frequency discriminator 9 to the terminal A, and the voltage at the terminal D is increased so that the reception is no longer possible of the television signal that has been received by the electronic tuner 2. Accordingly, the electronic tuner 2 stops production of the i-f signal, and the signal voltage a applied to the terminal A is changed to greatly differ from the voltage E.sub.1. During this course, the output signal of the Schmitt circuit 12 is changed to be at "0" level. Also, with the vanishment of the i-f signal the sync signal also vanishes, so that the output signal at the terminal X of the sync discriminator circuit 13 is changed to be at "0" level. With a further increase of the voltage at the terminal D, the electronic tuner 2 comes to receive a television signal of a different channel to produce again the i-f signal. As a result, the voltage E.sub.2 is applied again to the terminal A to cause the output signal of the Schmitt circuit 12 to be at "1" level and also the output signal at the terminal X of the sync separator circuit 13 to be at "1" level, thus resetting the SRFF 21 to render the output signal thereof to be at "0" level. After the inversion of the output signal of the SRFF 21 to the "0" level signal, the constant current circuit 25 is gain controlled according to the voltage a applied to the terminal A.
When the pushbutton switch 28 is closed, the SRFF 22 is set, and the output signal of the NOR gate 17 is caused to be at "0" level irrespective of the voltage applied to the terminal A, thus cutting off the constant current circuit 25. As a result, the voltage at the terminal D is gradually reduced for searching a television signal of a different channel.
As has been shown, the channel search and selection system based upon a voltage synthesizer is simple in construction and can be readily provided with the automatic frequency control function, so that it is extensively used in practice. However, this system is not provided with any means for storing the voltage appearing at the terminal D, i.e., the tuning voltage supplied to the electronic tuner 2. Therefore, when the television signal being received temporarily vanishes or when the intensity of the television signal is temporarily reduced, the tuning voltage, i.e., voltage at the terminal D, is readily changed. If this happens, it is liable to occur that the electronic tuner 2 can no longer correctly receive the television signal even with a subsequent interception of the television signal by the antenna 1. This is because without the i-f signal supplied to the frequency discriminator 9, the circuit 9 generates a specific output voltage V.sub.0, which is applied to the channel selection voltage generator circuit 10. If this voltage V.sub.0 is higher than the threshold voltage of the buffer circuit 11, the voltage at the terminal D is increased, while if it is lower than the threshold voltage the voltage at the terminal D is reduced. This results in the reception of a television signal of another channel as in the case when the pushbutton switch 27 or 28 is closed.
Furthermore, since sync discriminator circuit 13 and Schmitt circuit 12 are both provided for the purpose of ensuring accurate determination of the operation state of the electronic tuner 2 such that the tuner 2 receives a television signal, one of them may be omitted.